Theoretical study of the linear and nonlinear optical properties of conjugated systems

Abstract

Hyperpolarizabilities of conjugated ?-electron systems within and correlated Pariser–Parr–Pople (PPP) Hamiltonian have been studied. The polarizabilities of polyenes have been used to estimate the shift in optical gaps in the solid state using a charge–dipole approximation. It has been found that while this approximation gives a good estimate of the shift in ground state energies, the shifts in the dipole-allowed excited energies are smaller and hence give a smaller estimate of the optical gaps. The model exact energy and polarizability calculations on unsubstituted polyenes show that the dipole-allowed excited state exhibits sudden polarization, which is a consequence of one of the excited states being degenerate. In the case of substituted polyenes, states have demonstrated that both are double-bonded. A calculation of the NLO properties of substituted polyenes shows that while there is a large extent of contribution of the efficient electrons to molecular polarizability, the contribution of the SHG coefficients of the same substituted polyenes with more than one donor and acceptor groups shows inadequacy of the SHG coefficients. The reason for the SHG coefficients being placed along from one push–pull of the polyenes are enhanced when all groups are from the calculation of SHG coefficients on the geometry of the polyene. Applications to DFN3H (PDB) show that unsubstituted polyenes and states that the coefficients are dependent on the length, strength, direction, and the case of the THG power coefficients. The effect of static field on the SKG coefficients is very similar to those of the EFISH coefficients, push–pull dependence and the case strength found to be weaker. However, the various coefficients show that the presence of nitrogen atoms has a very large change in the THG increase and reduces it in unsubstituted polyenes calculated. For the substituted polyenes, coefficients comparison of various NLO properties shows that the Pockels coefficients are larger than the EFISH coefficients, which in turn are larger than the THG coefficients.